The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Referring now to FIG. 1, an exemplary control system 10 includes an input signal X (i.e. a target response of the control system 10), an output response signal Y, and an error signal E. A control module 12 controls the output response signal Y based on the error signal E. The error signal E is a difference between the input signal X and the output response signal Y. In other words, the control module 12 attempts to control the output response signal Y to “follow” the input signal X. The control system 10 can be described as:Y=E*H; andE=X−Y, where H represents a transfer function of the control system 10. Each of the values X, Y, and E is a vector that includes values that occur over time.
Referring now to FIG. 2, a control system 14 typically includes a disturbance N, such as noise. For example, the disturbance N may be introduced into the control system 14 due to performance, manufacturing, and/or environment imperfections. The disturbance signal N is added to the output response signal Y to model the effects of the disturbance N on the control system 14. The result is a final observable response signal Z. The control system 14 can be described as:Y=E*H; Z=Y+N; andE=X−Z. In other words, due to the presence of the disturbance signal N, the output response signal Y is not observable.
The disturbance signal N may include a random component (for example, random noise), a shock component, and/or a repeatable component (for example, a recurring disturbance due to a constant system imperfection). Typically, effects of a random disturbance signal N are non-correctable. For example, mitigating the effects of a random disturbance signal N may be limited to selecting a different transfer function H and/or changing other parameters of the control system.
Referring now to FIG. 3, a magnetic hard disk drive (HDD) may include a hard disk assembly (HDA) 15 that includes a voice-coil motor (VCM) 16 and head gimble assembly (HGA) 17. The VCM/HGA 16, 17 is supported by a hinge (not shown) that allows the VCM/HGA 16, 17 to swing. The rotational swing of the VCM/HGA 16, 17 controls the position of a read/write head 18. Rotational shock experienced by the HDD may adversely impact the VCM/HGA 16,17.
The VCM/HGA 16, 17 includes one or more hard drive platters 19 that include magnetic coatings that store servo and user data as magnetic fields. The platters 19 are rotated by a spindle motor that is schematically shown at 20.
Referring now to FIG. 4, positioning a read/write head over a disk medium is accomplished by having the VCM/HGA 16, 17 lock to a predetermined servo wedge. A servo wedge 21 may include several data fields, including a preamble field 22, a servo sync-mark (SSM) 24 field, a track/sector identification (ID) field 26, a plurality of position error signal (PES) fields 28, 29, and repeatable run-out (RRO) fields 30, 31.
The preamble field 22 may be used to synchronize the servo information stored in the servo track/sector ID field 26. The SSM 24 may be used to mark the ending point of the preamble field 22 and the starting point of the track/sector ID 26. The SSM 24 may also be used as a reference point for the position of other data payloads throughout servo fields. The track/sector ID field 26 indicates both the circumferential position and the coarse radial position of the read/write head 18. The track/sector ID field 26 typically includes a servo track number, which identifies the current track the read/write head 18 is located over while the read/write head 18 is seeking to a selected track.
The position information contained in a servo field is used to determine the fine position of the read/write head 18 on the platter surface and to provide a VCM control module (not shown) an instantaneous position error signal (PES). The PES 28, 29 provide information concerning fine radial positioning of the head. Typically, a PES is the difference between the measured position computed from the servo pattern and the desired position of the read/write head 18. RRO is a repeatable component in the disturbance signal N that may be caused by imperfections in the HDD and/or the platter 19. RRO may cause errors in position detection and control of the read/write head 18.